Fix for missing fee bump result (issue 388)

[jopmiddelkamp]

Fix for issue 388.

Also fixed the fee bump fee according to the Stellar documentation. https://developers.stellar.org/docs/encyclopedia/fee-bump-transactions#fee

It states that the fee set on the fee bump transaction should be the amount of stopen that you would like to pay for executing only the fee bump operation which is 1 operation. So this should be independent from the inner transaction.

Types of changes

• [x] Bug fix (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 change)
• [ ] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[jopmiddelkamp]

@elucidsoft, could you check this PR?