Headers should be case insensitive

[cesine]

Hi @fehguy we've been using swagger-client for about a year, its a great project! We noticed that our headers weren't being passed if they didn't match the case of the swagger spec.

I added some docs to #840 Nick's PR, let us know if there is anything else we can do to help get this merged in!

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html

HTTP header fields, which include general-header (section 4.5), request-header (section 5.3), 
response-header (section 6.2), and entity-header (section 7.1) fields, follow the same generic format 
as that given in Section 3.1 of RFC 822 [9]. Each header field consists of a name followed by a 
colon (":") and the field value. Field names are case-insensitive.

[fehguy]

Hi I merged #840 so if you want to move any other tests or logic from this PR into a new one, I'd be happy to add.

[cesine]

will do, its mostly just docs.

thanks @fehguy we really appreciated it!

[cesine]

@fehguy I squashed and rebased and pushed to this branch if you want to re-use this PR you can re-open it

[fehguy]

@cesine looks like it won't let me reopen it :(