Add crow::url_decode() function for decoding from percent-encoding.

docs/guides/routes.md

@@ -20,6 +20,19 @@ CROW_ROUTE(app, "/add/<int>/<int>")
 ```
 you can see the first `<int>` is defined as `a` and the second as `b`. If you were to run this and call `http://example.com/add/1/2`, the result would be a page with `3`. Exciting!
 
+Another example is to ask for the first part in a string, and the remainder in a path.
+Note that strings are in URL-encoding (aka Percent-encoding, eg `"image%20file.png"`).
+To decode into UTF-8, use `crow::decode_url(...)` to in-place decode the string.
+```cpp
+CROW_ROUTE(app, "/combine/<string>/<path>")
+([](std::string first, std::string remainder)
+{
+    // first can remain percent-encoded: perhaps we know for certain that it will always be simple text
+    crow::decode_url(remainder);
+    return first + " --> " + remainder;
+});
+```
+
 ## Methods
 You can change the HTTP methods the route uses from just the default `GET` by using `method()`, your route macro should look like `CROW_ROUTE(app, "/add/<int>/<int>").methods(crow::HTTPMethod::GET, crow::HTTPMethod::PATCH)` or `CROW_ROUTE(app, "/add/<int>/<int>").methods("GET"_method, "PATCH"_method)`.
 

include/crow/http_request.h

@@ -31,6 +31,22 @@ namespace crow // NOTE: Already documented in "crow/app.h"
         return empty;
     }
 
+    /// URLs given to routes are in URL-encoding (aka Percent-encoding)
+    /// eg "image file.png" --> "image%20file.png"
+    /// This operates on the string in-place and decodes a percent-encoded string to UTF-8
+    /// The resulting string will always be shorter than the original, so this function
+    /// will set a new null terminator at the end of the string.
+    inline int url_decode( char* url )
+    {
+        return qs_decode(qs);
+    }
+
+    /// This will decode the url in-place, resizing the string to the new shorter size.
+    inline void url_decode( std::string& url )
+    {
+        url.resize( url_decode(&url[0]) );
+    }
+
     /// An HTTP request.
     struct request
     {

tests/unittest.cpp

@@ -4116,3 +4116,10 @@ TEST_CASE("option_header_passed_in_full")
     CHECK(res.find(ServerName) != std::string::npos);
     app.stop();
 }
+
+TEST_CASE("url_decoding")
+{
+    std::string url = "image%20file.png";
+    crow::url_decode(url);
+    CHECK(url == "image file.png");
+}