add proxy support for http client

[Guest0x0]

This PR adds CONNECT based proxy support to @http.Client and the various request methods. A new optional argument proxy? : @http.Client is added to @http.Client::new and the various request helpers. When present, proxy should be a @http.Client in a clean state (i.e. not in the middle of a request). The new client will send a CONNECT request to the proxy client. If the proxy client response with a 2XX response code, the proxy client will be set to pass-through mode, and all subsequent traffic in the new client will go through the proxy client tunnel.

The merits of using another @http.Client as proxy (compared to #191):

• https support (the CONNECT request itself is encrypted) come for free
• proxy authentication is easy to perform: just set extra headers for the proxy client

Some examples can be found in src/http/proxy_test.mbt.

cc @Milky2018

[coveralls]

Pull Request Test Coverage Report for Build 828

Details

• 46 of 52 (88.46%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 79.379%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/http/parser.mbt	5	6	83.33%
src/http/client.mbt	25	30	83.33%

Totals
Change from base Build 826:	0.7%
Covered Lines:	1432
Relevant Lines:	1804

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💛 - Coveralls

[Milky2018]

Excellent work! Thanks for CC-ing me.