case-insensitive URI scheme comparison

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

src/playlist/plugins/SoundCloudPlaylistPlugin.cxx

@@ -24,6 +24,7 @@
 #include "config/Block.hxx"
 #include "input/InputStream.hxx"
 #include "tag/TagBuilder.hxx"
+#include "util/ASCII.hxx"
 #include "util/StringCompare.hxx"
 #include "util/Alloc.hxx"
 #include "util/Domain.hxx"
@@ -68,7 +69,7 @@ soundcloud_resolve(const char* uri)
 {
 	char *u, *ru;
 
-	if (StringStartsWith(uri, "https://")) {
+	if (StringStartsWithCaseASCII(uri, "https://")) {
 		u = xstrdup(uri);
 	} else if (StringStartsWith(uri, "soundcloud.com")) {
 		u = xstrcatdup("https://", uri);
@@ -273,7 +274,7 @@ try {
 static SongEnumerator *
 soundcloud_open_uri(const char *uri, Mutex &mutex, Cond &cond)
 {
-	assert(strncmp(uri, "soundcloud://", 13) == 0);
+	assert(StringEqualsCaseASCII(uri, "soundcloud://", 13));
 	uri += 13;
 
 	char *u = nullptr;