Feature/unordered map conversion

[theodelrieu]

Add conversion for std::unordered_map, when KeyType != basic_json<>::string_t

Fixes #1133

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

[coveralls]

Coverage remained the same at 100.0% when pulling 2c920a1 on theodelrieu:feature/unordered_map_conversion into e830bc5 on nlohmann:develop.

[gregmarr]

Weird that the single header contains things that aren't part of your PR. I guess someone committed to develop without running make amalgamate recently.

[gregmarr]

LGTM

[nlohmann]

Looks good to me.

[nlohmann]

Thanks @theodelrieu for the PR and @gregmarr for the review!

[014972304505347]

When can we expect the dev branch to merge with master branch? I use vcpkg to install this library -- is there a way to get dev branch using vcpkg?

[nlohmann]

I hope to release version 3.1.3 before end of July.