accounts/abi: Negative numbers not properly converted in ABI encoding

When converting a negative number e.g., -2, the resulting ABI encoding
should look as follows:
fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe.
However, since the check of the type is for an uint instead of an
int, it results in the following ABI encoding:
0101010101010101010101010101010101010101010101010101010101010102. The
Ethereum ABI
(https://github.com/ethereum/wiki/wiki/Ethereum-Contract-ABI) says,
that signed integers are stored in two's complement which should be
of the form ffffff.... and not 01010101..... for e.g. -1. Thus, I
removed the type check in numbers.go as well as the function S256
as I don't think they are correct. Or maybe I'm missing something?

accounts/abi/method.go

@@ -62,7 +62,7 @@ func (m Method) pack(method Method, args ...interface{}) ([]byte, error) {
 			// calculate the offset
 			offset := len(method.Inputs)*32 + len(variableInput)
 			// set the offset
-			ret = append(ret, packNum(reflect.ValueOf(offset), UintTy)...)
+			ret = append(ret, packNum(reflect.ValueOf(offset))...)
 			// Append the packed output to the variable input. The variable input
 			// will be appended at the end of the input.
 			variableInput = append(variableInput, packed...)