If the header is not specified or there is no proper format, text/plain is used. 
    2.   // Accept: text/plain
    3.   c.Format("Hello, World!")
    4.   // => Hello, World!
    5.   // Accept: text/html
    6.   c.Format("Hello, World!")
    7.   // => <p>Hello, World!</p>
    9.   c.Format("Hello, World!")
    10.   // => "Hello, World!"
    12. })