Allow for an empty contract in ERC20Transfers function

[Benjamin F Knigge]

pull request #10

This would allow for an empty string pointer to be passed in to ERC20Transfers
resulting in the parameter of the contract address being omitted from the underlying api call.

The result being all coin transfers being returned instead of a list filtered by contract address.

[Zhennan LI]

Have you tried passing nil to contractAddress? I think that would do what you need.

[Benjamin F Knigge]

yes, nil seems to work