Add function to expose allowed methods for use in custom 405-handlers

Fixes go-chi#870

Author: flimzy

Diff for: context.go

@@ -133,6 +133,18 @@ func (x *Context) RoutePattern() string {
 	return routePattern
 }
 
+// WithRouteContext returns the list of methods allowed for the current
+// request, based on the current routing context.
+func (x *Context) AllowedMethods() []string {
+	result := make([]string, 0, len(x.methodsAllowed))
+	for _, method := range x.methodsAllowed {
+		if method := methodTypString(method); method != "" {
+			result = append(result, method)
+		}
+	}
+	return result
+}
+
 // replaceWildcards takes a route pattern and recursively replaces all
 // occurrences of "/*/" to "/".
 func replaceWildcards(p string) string {

Diff for: context_test.go

@@ -1,6 +1,9 @@
 package chi
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 // TestRoutePattern tests correct in-the-middle wildcard removals.
 // If user organizes a router like this:
@@ -91,3 +94,26 @@ func TestRoutePattern(t *testing.T) {
 		t.Fatalf("unexpected non-empty route pattern for nil context: %q", p)
 	}
 }
+
+func TestAllowedMethods(t *testing.T) {
+	t.Run("expected methods", func(t *testing.T) {
+		want := "GET HEAD"
+		rctx := &Context{
+			methodsAllowed: []methodTyp{mGET, mHEAD},
+		}
+		got := strings.Join(rctx.AllowedMethods(), " ")
+		if want != got {
+			t.Errorf("Unexpected allowed methods: %s, want: %s", got, want)
+		}
+	})
+	t.Run("unexpected methods", func(t *testing.T) {
+		want := "GET HEAD"
+		rctx := &Context{
+			methodsAllowed: []methodTyp{mGET, mHEAD, 9000},
+		}
+		got := strings.Join(rctx.AllowedMethods(), " ")
+		if want != got {
+			t.Errorf("Unexpected allowed methods: %s, want: %s", got, want)
+		}
+	})
+}