Encoding of mixed dynamic/fixed arguments fails

[markspanbroek]

Using both FixedBytes and DynamicBytes arguments in a function call doesn't work due to an incorrect encoding.

For example, calling this solidity function will trigger an incorrect encoding:

function mixedArguments(uint integer, bytes32 fixedbytes, address addr, bytes memory dynamicbytes)

For more details, a failing test scenario, and inspiration on how to fix, please see #44.