…he RFC 7807 specification.

ApiResponse property ContentHeaders

Implement RFC 7807 to support Problem Details in bad validation requests

- Added IContentSerializer, JsonContentSerializer, XmlContentSerializer
- RefitSettings allows the IContentSerializer to be specified
- Refit defaults to JsonContentSerializer for back-compat
- Tests modified to exercise Json and Xml Serialization
- .NET Standard 1.4 has package dep on System.Xml.XmlSerializer

 - Added Obsolete attributes
 - Added Serialized enum as a generic replacement of JSON enum
 - Updated tests to use ContentSerializer property
 - Updated README with XML example

Conflicting PreserveAttribute when using InternalsVisibleTo

IContentSeriaizer
- all methods made async
- removed 2nd deserialize method
- type safe operation

ApiException
- Added GetContentAsAsync<T>() method
- Marked GetContentAs<T>() as obsolete

…Exception.Content

 When form encoding objects that have a property that is an IEnumerable type, use the QueryAttribute and it's CollectionFormat to determine how to encode the object.

 In general, this feature could be fleshed out much more deeply. Future work could include respecting custom formats and delimiters specified on the QueryAttribute, as well as support for collections stored in dictionaries that are form encoded.

Expand XMLDocs for QueryAttribute and CollectionFormat

- Braces on new lines
- Rename FormValueDictionary -> FormValueMultimap

- Remove FormValueMultimap indexer. This had a larger than necessary risk of being misused and introducing bugs. LINQ extensions can more expressively verify functionality in the tests.

This ought to help developers diagnose where the error occurred. The current stack trace provides no actionable information and requires going through the whole Interface searching for violations.

```
Unhandled Exception: System.ArgumentException: URL has parameter member, but no method parameter matches
   at Refit.RestMethodInfo.BuildParameterMap(String relativePath, List`1 parameterInfo) in D:\a\1\s\Refit\RestMethodInfo.cs:line 143
   at Refit.RestMethodInfo..ctor(Type targetInterface, MethodInfo methodInfo, RefitSettings refitSettings) in D:\a\1\s\Refit\RestMethodInfo.cs:line 64
   at Refit.RequestBuilderImplementation`1..ctor(RefitSettings refitSettings) in D:\a\1\s\Refit\RequestBuilderImplementation.cs:line 58
   at Refit.RequestBuilderFactory.Create[T](RefitSettings settings) in D:\a\1\s\Refit\RequestBuilderFactory.cs:line 17
   at Refit.RequestBuilder.ForType[T](RefitSettings settings) in D:\a\1\s\Refit\RequestBuilder.cs:line 24
   at Refit.RestService.For[T](HttpClient client, RefitSettings settings) in D:\a\1\s\Refit\RestService.cs:line 28
   at Refit.RestService.For[T](HttpClient client) in D:\a\1\s\Refit\RestService.cs:line 33
   at MyCode.Program.Main(String[] args) in /Users/me/Documents/MyCode/MyCode/Program.cs:line 16
```

Isolated Content Serialization

Provide additional information during errors

 Add support for url form encoding collections

Options Attribute

Exclude from Obfuscation

Update depenencies

…needed to run the targets

Fixes #598

Update core

…ash-from-hosturl

Trim trailing `/` from hostUrl

…ception-if-hosturl-isnullorwhitespace

614 should throw argumentexception if hosturl isnullorwhitespace

… and build directories.

Fixes #619