fix: support method call from structures for string-format

[rmarku]

This MR fix issue #839. Here the called object method is inside a structure (common pattern for dependency injection). Before the rule was not running for this rules.

The only thing is the function scope is tied to the structure name of the object, not to the object name.

This MR tries to fix this. A test was added to the string-format to evaluate this.

• Did you add tests?
• Does your code follows the coding style of the rest of the repository?
• Does the Travis build passes?

Closes #839

[chavacava]

Hi @rmarku thanks for the PR!

(abrazo de un santiagueño a un tucumano)