String type keys aren't correct delimited in type_base::m_edit_link

[royoung]

tried out generated proxy with schema that had string type as entity key. At runtime the m_edit_link was not set correctly:

"http://serviceroot/entitySet(fookey)" where as should have been "http://serviceroot/entitySet('fookey')"

[royoung]

Should be able to fix in ::utility::string_t odata_entity_value::get_entity_key_string
(also check primitive type for string and delimit accordingly)