Ordered map remove duplicate codes

[Tomerkm]

[Describe your pull request here. Please read the text below the line, and make sure you follow the checklist.]
I removed the code at the function:
std::pair<iterator, bool> emplace(const key_type& key, T&& t)
and replaced which a method that exists at the class and do the job.

The method find do the same as the iterate at the code does.
Instead of writing the same code, I have called the method find.
The idea is to use the function does the job instead of rewriting the same code.

I want to see if this ok before I change on the others.

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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 files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @Tomerkm
Please read and follow the Contribution Guidelines.

[coveralls]

coverage: 100.0%. remained the same when pulling 87ae0e6 on Tomerkm:ordered_map_remove_duplicate_codes into 5d27543 on nlohmann:develop.

[nlohmann]

Please amalagamate the source code.

[nlohmann]

(Also, please update from the develop branch as it has a working CI now.)