[cpp-restsdk] Update json double/float parse.

[adameste]

Fix cpp-restsdk double and float parse.

double.NaN was parsed when server doesn't ensure the floating point value has decimal point in in.

Fixed incorrect server response json parsing. For example the following json:

{
    "double_value": 42
}

As of now, the value 42 is parsed as NaN, because json library returns false on is_double().
Additional condition ensures NaN is not returned when not needed.
Implemented for both float & double data types.

[wing328]

Thanks for the PR. Please follow the instruction in the PR template below to update the samples so that CI can test the change:

• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.

cc @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)

[adameste]

Ran all three above mentioned commands & commited changes.