This member is added by an implicit conversion from If-Modified-Since to
any2stringadd[If-Modified-Since] performed by method any2stringadd in scala.Predef.
This member is added by an implicit conversion from If-Modified-Since to
StringFormat[If-Modified-Since] performed by method StringFormat in scala.Predef.
RFC 7231 section 3.3
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since