Deprecate :read_write option on HTTP.open?/2

[ericmj]

Closes #410.

[coveralls]

Pull Request Test Coverage Report for Build 936c36337beeef8226e11a258785a80d020df527-PR-426

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• 32 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.3%) to 87.818%

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Files with Coverage Reduction	New Missed Lines	%
lib/mint/core/transport/tcp.ex	1	95.65%
lib/mint/unsafe_proxy.ex	1	34.21%
lib/mint/http.ex	13	62.86%
lib/mint/core/transport/ssl.ex	17	87.22%

Totals
Change from base Build 74e0ec62d9efe26409fdb7c40181958291a72f3f:	0.3%
Covered Lines:	1276
Relevant Lines:	1453

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💛 - Coveralls

[whatyouhide]

Left only one comment, LGTM otherwise! 👏