Fix an Xcode 6 warning

Building on Xcode 6 results in the following warning for NSDictionary+BDBOAuth1Manager.m:
"Convenience initializer missing a 'self' call to another initializer"

Since the category's init method is only used in the adjacent class method, I collapsed the two methods into one.

(Another approach would be to use Clang #pragma directives to suppress the warning.)