feat!: Headers should be `Headers`

[danielbankhead]

Headers should be Headers

• creates consistency throughout the libraries; no need to check properties for different casing
• Alignment with Request#headers
• Clients or intermediates can easily append an existing header or create it if it does exist, like so:

const headers = new Headers();

// creates, if not exist, or appends to an existing header
headers.append('x-goog-api-client', 'abc');

• Fixes: #262

🦕