Parses headers parameters. Keys are case insensitive and downcased, invalid key-value pairs are discarded. ## Examples iex> params("foo=bar") %{"foo" => "bar"} iex> params(" foo=bar ") %{"foo" => "bar"} iex> params("FOO=bar") %{"foo" => "bar"} iex> params("Foo=bar; baz=BOING") %{"foo" => "bar", "baz" => "BOING"} iex> params("foo=BAR ; wat") %{"foo" => "BAR"} iex> params("=") %{}