JAV-38 Inner transaction singer delegation

[rg911]

• Changed signer param to be optional on toAggregate when adding inner transactions. If not signer in inner transaction not provided, the main Aggregate transaction signer will be delegated.
• This change ONLY applies on AggregateComplete transaction
• Added validation if no signer (inner tx) provided in AggregateBond transaction
• Added tests

[rg911]

@dgarcia360 @evias

[coveralls]

Pull Request Test Coverage Report for Build 753

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 748:	0.0%
Covered Lines:	10
Relevant Lines:	10

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💛 - Coveralls