Don't modify the client gateway url struct in client methods

[welteki]

Description

The Client is not safe for concurrent use as each method modifies the gateway URL path.
This was fixed by creating a new URL struct from the base URL before manipulating the URL.

Motivation and Context

• I have raised an issue to propose this change (required)
• My issue has received approval from the maintainers or lead with the design/approved label

Make the client safe for concurrent use.

This issues was discovered while working on a new feature to invoke functions using the client.

How Has This Been Tested?

All unit tests passed.

Types of changes

• 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)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

Approved