Make '<String as FromParam>::Error' 'Infallible'.

Fixes #1679.

core/codegen/tests/typed-uris.rs

@@ -427,7 +427,7 @@ struct Third<'r> {
 #[post("/<foo>/<bar>?<q1>&<rest..>")]
 fn optionals(
     foo: Option<usize>,
-    bar: Result<String, &'_ str>,
+    bar: Option<String>,
     q1: Result<usize, Errors<'_>>,
     rest: Option<Third<'_>>
 ) { }

core/lib/src/request/from_param.rs

@@ -193,7 +193,7 @@ impl<'a> FromParam<'a> for &'a str {
 }
 
 impl<'a> FromParam<'a> for String {
-    type Error = &'a str;
+    type Error = std::convert::Infallible;
 
     #[inline(always)]
     fn from_param(param: &'a str) -> Result<String, Self::Error> {