Header parsing assumes whitespace after colon

[stewartbright]

RFC7230 (HTTP 1.1) 3.2 "Header fields" specifies that whitespace between colon and value is optional, but http_client_asio::read_headers started reading the value at one after the colon, assuming there would always be exactly one space.

For an HTTP server that doesn't add whitespace between colon and value, cpprestsdk reads an incorrect value and can read a truncated response, or try to read one that is 2^64 in the case where Content-Length has a single digit value.

Examples:

Content-Length:6

• value is read as an empty string and interpreted as content of 2^64 in length.

Content-Length:26

• value is read as 6 and truncated response is read.

[msftclas]

Hi @stewartbright, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;

[ras0219-msft]

Thank you for this patch. It has been merged into the development branch.