Treat JsonRpc.GasCap=0 as max cap

[rubo]

Fixes #4360

Changes:

The JsonRpc.GasCap is now handled like in Geth, i.e. when it's 0, then the gas limit is set to max.

Types of changes

What types of changes does your code introduce?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

Comments about testing , should you have some (optional)

Further comments (optional)

The change is in TransactionForRpc.cs. Other files have been modified due to a small refactoring.