Improve parse_ubjson_fuzzer

[tanuj208]

I noticed that in parse_ubjson_fuzzer, the method json::to_ubjson() was called with default values to the arguments use_size and use_type. I went through the coverage data and found out that there were many lines of code that could be covered if provided other combinations of values to them.

So, I added code with changing the values to these arguments and compared the resulting vectors. By doing this coverage went up by
Line coverage = ~240 lines
Functional coverage = 18 functions
Region coverage = ~90 regions

Coverage report

Let me know if there is an incorrect use of the API.

[coveralls]

Coverage remained the same at 100.0% when pulling c3c5c03 on tanuj208:develop into 27f5a6e on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks, the changes look good. I'll merge once the CI runs through.