fix(ajax): Fix case-insensitive headers in HTTP request

[DostalTomas]

Description:
In RFC 7230 and RFC 7540 is written, that HTTP headers is case-insensitive, so this is fix for correct serializing request body based on HTTP headers.

[coveralls]

Pull Request Test Coverage Report for Build 7913

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 96.969%

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Totals
Change from base Build 7909:	0.003%
Covered Lines:	5791
Relevant Lines:	5972

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💛 - Coveralls

[benlesh]

1. Do we have an example of where the current implementation fails?
2. Can we please add a test to confirm that the changes got the desired effect?

[DostalTomas]

Thank You for your response.
Test has been added and branch squashed.
I get this issue, when I set content-type: application/json; charset=UTF-8 header and on server I got URL-encoded data. I know, that this is edge case, but it happens.

const reqSettings = {
   url: 'some/url',
   headers: {
      'content-type': 'application/json; charset=UTF-8'
   },
   body: {
      hello: 'world'
   }
}

ajax(reqSettings).subscribe();

[benlesh]

Thanks, @DostalTomas!

[DostalTomas]

Thank You too ;)