Improve ordered map

[Tomerkm]

[Describe your pull request here. Please read the text below the line, and make sure you follow the checklist.]

1. I updated the following method: T& at(const key_type& key)

it will call to the following method: const T& at(const key_type& key) const The idea is to remove duplicate code

2. For each function that has a loop call to the method this->end() several times, I did the following changes: For preventing it be called several times, the returned variable is stored in a variable.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• [x] Changes are described in the pull request, or an existing issue is referenced.
• [x] The test suite compiles and runs without error.
• [x] Code coverage is 100%. Test cases can be added by editing the test suite.
• [x] 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[bot]]

🔴 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 5190c4d643a519606965feba37e0dab57a2fead1 on Tomerkm:improve_ordered_map into 5d2754306d67d1e654a1a34e1d2e74439a9d53b3 on nlohmann:develop.

[Tomerkm]

Please add a summary of the changes you made and the reason for the changes.

I added summary and explain why I did the changes. Thanks