fix send error header sometime and add collectAllHeaders() function

[kingxf1995]

Fix send error header when url is like "http://github.com" (not end with '/').
Add collectAllHeaders() to support collecting all response headers. Old collectHeaders() can not handle two header with the same name.

[codecov-io]

Current coverage is 27.80% (diff: 100%)

Merging #2713 into master will not change coverage

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  Lines          3625       3625          
  Methods         335        335          
  Messages          0          0          
  Branches        656        656          
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  Hits           1008       1008          
  Misses         2441       2441          
  Partials        176        176          

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[kingxf1995]

If use this code

httpclient.begin("http://xxx.xxx.xxx");
code = httpclient.GET();

The httpclient will send

GET HTTP/1.1
Host: xxx.xxx.xxx
User-Agent: ESP8266HTTPClient
Connection: close
Accept-Encoding: identity;q=1,chunked;q=0.1,*;q=0

But it is a bad request, after fix httpclient will send

GET / HTTP/1.1
Host: xxx.xxx.xxx
User-Agent: ESP8266HTTPClient
Connection: close
Accept-Encoding: identity;q=1,chunked;q=0.1,*;q=0

[earlephilhower]

Thanks for your PR, but the core and libraries have changed enough that this PR now has a merge conflict.

Could you merge it manually with the latest core, so we can consider it for future releases?