Fix cfg for test

Fix up cfg attributes to work on an xor basis.

Previously, the cfg(any()) attributes would cause issues when
both native-tls and tls features were enabled. Now, https functions
and enum variants will only be created when tls xor native-tls are
enabled. Additionally, a compile error has been added for when
both tls and native-tls features are enabled.

src/lib.rs

@@ -181,6 +181,12 @@ pub fn patch(path: &str) -> Request {
     request("PATCH", path)
 }
 
+// Compilation error when both tls and native-tls features are enabled
+#[cfg(all(feature = "tls", feature = "native-tls"))]
+std::compile_error!(
+    "You have both the \"tls\" and \"native-tls\" features enabled on ureq. Please disable one of these features."
+);
+
 #[cfg(test)]
 mod tests {
     use super::*;