[C++][Pistache] fix missing default values for string and enum types

Created by: JulianGmp

The current generated codes for the C++ pistache server does not pass the correct default value for strings (and thus enums) to the templates. This PR just fixes that.

That way, non-required enum values are not set to an empty string in the constructor of a model, if a corresponding default value is specified.

yaml example

Pet:
  type: object
  required:
    - someEnum
    - someOtherEnum
    - someString
    - someOtherString
  properties:
    someEnum:
      type: string
      enum: [EnumVal1, EnumVal2, EnumVal3]
      default: EnumVal1
    someOtherEnum:
      type: string
      enum: [OtherEnumVal1, OtherEnumVal2, OtherEnumVal3]
    someString:
      type: string
      default: "hello world"
    someOtherString:
      type: string

generated constructor:

Pet::Pet()
{
    m_SomeEnum = "EnumVal1";
    m_SomeOtherEnum = "";
    m_SomeString = "hello world";
    m_SomeOtherString = "";
}

Please note that the someOtherEnum value still receives an invalid value, since no valid default was defined!

NOTE: I did run the generate-samples script, but it did not generate differing files. I suspect that the samples don't have this edge case anywhere.

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