Add support for "application/octet-stream" content type

Added support for "application/octet-stream" by making sure that HttpBodyContent converts dynamic body to content and content to dynamic body consistently using specified encoding. 

Made HttpBodyContent and bunch of related classes/interfaces internal, as they could not be swapped by 
developers and therefore should not be visible. Delete a few instances of [Obsolete] attribute and replaced those with internal constructors.