fix: Add ToString() override to TwiML class

[Swimburger]

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Fixes

Currently, there's a ToString method with an optional SaveOptions formattingOptions parameter.
However, this is not overriding the default ToString method that all objects inherit.

As a result, in C#, when TwiML is cast to object, the ToString method will return the name of the class, for example Twilio.TwiML.MessagingResponse. This probably doesn't occur much in the wild.

However, in F#, the Object.ToString() method will always be used if you don't specify the formattingOptions argument.
Therefor, the Object.ToString() method should be overriden inside TwiML.

// before
VoiceResponse().Say("Ahoy!").ToString()                  // outputs Twilio.TwiML.VoiceResponse
VoiceResponse().Say("Ahoy!").ToString(SaveOptions.None)  // outputs TwiML correctly

// after
VoiceResponse().Say("Ahoy!").ToString()                  // outputs TwiML correctly
VoiceResponse().Say("Ahoy!").ToString(SaveOptions.None)  // outputs TwiML correctly

This PR takes care of that.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
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• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified