Version Bump: v3.5.6: PR sendgrid#98: Updated PHPDocs

Version Bump v3.5.5: PR sendgrid#95

CreateCurlOptions method regression tests

Version Bump:v3.9.4: sendgrid#96: Fix constructor function signature

Version Bump v3.9.3: sendgrid#93 Don't overwrite headers

Version Bump: v3.9.2: Fix sendgrid#12, union operator

PR sendgrid#89, Fixes sendgrid#88 restore prepareResponse function

Version Bump v3.9.0: Roll up Hacktoberfest Release

Version Bump v3.8.0: sendgrid#23: Automatically retry when rate limit…

… is reached

Version Bump v3.7.0: sendgrid#19 Added ability to get headers as asso…

…ciative array

Version Bump v3.6.0: Pull sendgrid#16 Pass the curlOptions to the cli…

…ent in buildClient