Therefore, the "null" value represents the null json element.

* getPropertiesAsJson returns all properties of the file.
* This is not the case of getPropertiesAsMap/AsVector methods...

…perty

Instead of an hardcoded value.

…Vector/Map/Json'

* FileProperties::asJson returns the format properties.
* FileProperties::allPropertiesAsJson returns all properties.
* It is to keep a symmetry between all 'properties' classes with their
'asJson' method.

…into dev_mediaPropertiesAsJson

Conflicts:
	src/AvTranscoder/properties/StreamProperties.cpp

* Added JsonStreamWriter::escapeJsonString to escape strings accordingly to the JSON standard.

* The error was: 'explicit specialization in non-namespace scope'.
* Solution: move the specializations of the member templates outside of
the class body.