As discussed in #74, we need to assure that all single-value headers we define have appropriate error handling for when more then one value is received. This could be a variety of things, including "it's not important for this header."

• Authorization
• Content-Language
• Content-Length
• Content-Location
• Content-Range
• Content-Type
• Date
• ETag
• Expect
• From
• Host
• If-Modified-Since
• If-Range
• If-Unmodified-Since
• Last-Modified
• Location
• Max-Forwards
• Range
• Referer
• Retry-After
• Server
• User-Agent
• Proxy-Authorization
• Age
• Expires