Expression of default values is always `cpp_unexposed_expression`

[foonathan]

• cppast version: latest one
• parser: libclang_parser
• clang version: 3.9.1+

The default value of a function parameter, template non type parameter and the initializer of a (member) variable is not exposed.
This is due to an ambiguity when parsing the children of the libclang cursor, there is no way to distinguish between the expression in the array size and the expression of the initializer.

Also see #7.