Let RUBY_BUILD_MIRROR_PACKAGE_URL work even when there is no checksum

* And add RUBY_BUILD_IGNORE_CHECKSUM when it is expected the alternative URL has different contents.

Author: eregon

bin/ruby-build

@@ -365,14 +365,18 @@ fetch_tarball() {
   if [ "$package_url" != "${package_url/\#}" ]; then
     checksum="${package_url#*#}"
     package_url="${package_url%%#*}"
+  fi
 
-    if [ -n "$RUBY_BUILD_MIRROR_URL" ]; then
-      if [[ -z "$RUBY_BUILD_DEFAULT_MIRROR" || $package_url != */cache.ruby-lang.org/* ]]; then
-        mirror_url="${RUBY_BUILD_MIRROR_URL}/$checksum"
-      fi
-    elif [ -n "$RUBY_BUILD_MIRROR_PACKAGE_URL" ]; then
-      mirror_url="$RUBY_BUILD_MIRROR_PACKAGE_URL"
+  if [[ -n "$RUBY_BUILD_MIRROR_URL" && -n "$checksum" ]]; then
+    if [[ -z "$RUBY_BUILD_DEFAULT_MIRROR" || $package_url != */cache.ruby-lang.org/* ]]; then
+      mirror_url="${RUBY_BUILD_MIRROR_URL}/$checksum"
     fi
+  elif [ -n "$RUBY_BUILD_MIRROR_PACKAGE_URL" ]; then
+    mirror_url="$RUBY_BUILD_MIRROR_PACKAGE_URL"
+  fi
+
+  if [ -n "$RUBY_BUILD_IGNORE_CHECKSUM" ]; then
+    checksum=""
   fi
 
   local tar_args="xzf"