Optimize canonicalizeContentType

For context, this function is run on every request within ServeHTTP, and
the mime parsing and now lowercasing, is a marginal, but non trivial
amount of memory allocations.

The expensive bit is hitting the mime.FormatMediaType path, when we
already have a canonical form.

This removes the most expensive part over arguable the most common cases
where there are no additional parameters on the Content-Type.

```
$ go test -v -bench '^BenchmarkCanonicalizeContentType$' -run '^$' .
goos: darwin
goarch: arm64
pkg: github.com/bufbuild/connect-go
BenchmarkCanonicalizeContentType
BenchmarkCanonicalizeContentType/simple
BenchmarkCanonicalizeContentType/simple-10         	 7160896	       157.4 ns/op	      48 B/op	       1 allocs/op
BenchmarkCanonicalizeContentType/with_charset
BenchmarkCanonicalizeContentType/with_charset-10   	 1780041	       674.4 ns/op	     424 B/op	       6 allocs/op
BenchmarkCanonicalizeContentType/with_other_param
BenchmarkCanonicalizeContentType/with_other_param-10         	 2029819	       592.6 ns/op	     424 B/op	       6 allocs/op
PASS
ok  	github.com/bufbuild/connect-go	5.129s
```

protocol.go

@@ -321,6 +321,10 @@ func canonicalizeContentType(ct string) string {
 		return ct
 	}
 
+	if len(params) == 0 {
+		return base
+	}
+
 	// According to RFC 9110 Section 8.3.2, the charset parameter value should be treated as case-insensitive.
 	// mime.FormatMediaType canonicalizes parameter names, but not parameter values,
 	// because the case sensitivity of a parameter value depends on its semantics.

protocol_test.go

@@ -29,6 +29,7 @@ func TestCanonicalizeContentType(t *testing.T) {
 	}{
 		{name: "charset param should be treated as lowercase", arg: "application/json; charset=UTF-8", want: "application/json; charset=utf-8"},
 		{name: "non charset param should not be changed", arg: "multipart/form-data; boundary=fooBar", want: "multipart/form-data; boundary=fooBar"},
+		{name: "no parameters should not have base normalized", arg: "APPLICATION/json;  ", want: "application/json"},
 	}
 	for _, tt := range tests {
 		tt := tt
@@ -38,3 +39,26 @@ func TestCanonicalizeContentType(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkCanonicalizeContentType(b *testing.B) {
+	b.Run("simple", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_ = canonicalizeContentType("application/json")
+		}
+		b.ReportAllocs()
+	})
+
+	b.Run("with charset", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_ = canonicalizeContentType("application/json; charset=utf-8")
+		}
+		b.ReportAllocs()
+	})
+
+	b.Run("with other param", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_ = canonicalizeContentType("application/json; foo=utf-8")
+		}
+		b.ReportAllocs()
+	})
+}