Fix missing suffix in fork mode without proxy

[webthethird]

Resolves #7

When using fork mode with a proxy, there are two implementations, each with a suffix V1 or V2, plus a single proxy without a suffix. In path mode there are two proxies as well, with the same suffixes. So when generating the low-level calls, we were first checking whether a proxy was given (to decide which contract variable name to use) and then checking if fork mode is being used (to determine whether to add the suffix to the proxy).

However, this did not consider the case where fork mode is used without a proxy. In that case we still have two implementations with suffixes, but since there is no proxy, it doesn't matter whether or not we're using fork mode - we need to include the suffixes anyway.

This was resolved by simply appending or proxy is None to the if condition on line 438 of code_generation.py