Output incorrect json file

**Describe the bug**

When I ran the test with "googletest/test/googletest-output-test_.cc" in this repository and output the json file as shown below, the comma was missing and the json file was incorrect.

[googletest-output-test_result.json.txt](https://github.com/google/googletest/files/8876263/googletest-output-test_result.json.txt)
line: 1687

**Steps to reproduce the bug**

```
$ git clone https://github.com/google/googletest.git -b release-1.11.0
$ cd googletest
$ mkdir build
$ cd build
$ cmake .. -Dgtest_build_tests=ON -Dgmock_build_tests=ON
$ make
$ cd googletest
$ ./googletest-output-test_ --gtest_output="json:googletest-output-test_result"
```

**Does the bug persist in the most recent commit?**

I have only tried it with release-1.11.0.

**What operating system and version are you using?**

debian

**What compiler and version are you using?**

```
$ clang -v
clang version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Candidate multilib: .;@m64
Selected multilib: .;@m64
```

**What build system are you using?**

```
$ cmake --version
cmake version 3.13.4

CMake suite maintained and supported by Kitware (kitware.com/cmake).
```