Make From param optional in Transaction object

[oilnam]

In eth_call the from field is optional (unlike for instance in eth_sendTransaction).

However, the current code always fills in the from field with an empty string if no actual value is provided, which results in the following error:

invalid argument 0: hex string has length 0, want 40 for common.Address

when using eth_call with no from field provided.

This fix makes the from field optional so that the rpc json request gets correctly serialized.

[coveralls]

Coverage increased (+0.004%) to 98.268% when pulling d977398 on oilnam:manlio-fix-call-params into 4a39f95 on onrik:master.

[oilnam]

@onrik any thoughts on this small PR? Thanks ;)