Upload: Only sign required params

[udipl]

Brief Summary of Changes

The Upload API request signing includes all passed parameters to the signing method, despite the documentation clearly states that All parameters added to the method call should be included except: file, cloud_name, resource_type and your api_key.
The server rejects signatures that include one of these values.

This breaks when a ResourceType field is non-empty in an UploadParams struct for a request; it gets marshaled as resource_type and included in the signature, which doesn't match the server's logic.

What does this PR address?

• GitHub issue Request params passed in for signing in Upload API request are not filtered in accordance with documentation #26
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I ran the full test suite before pushing the changes and all the tests pass.

[const-cloudinary]

@udipl, thank you for contribution!