Use RFC2396_PARSER instead of DEFAULT_PARSER

`URI::DEFAULT_PARSER` switched from `URI::RFC2396_PARSER` to
`URI::RFC3986_PARSER`, which complains about escape/unescape:

> /Users/dharsha/repos/json_schemer/lib/json_schemer.rb:116: warning: URI::RFC3986_PARSER.unescape is obsolete. Use URI::RFC2396_PARSER.unescape explicitly.
> /Users/dharsha/repos/json_schemer/lib/json_schemer.rb:254: warning: URI::RFC3986_PARSER.escape is obsolete. Use URI::RFC2396_PARSER.escape explicitly.

This switches back to `URI::RFC2396_PARSER` with a fallback to
`URI::DEFAULT_PARSER` for older Ruby versions.

Related:
- https://bugs.ruby-lang.org/issues/19266
- ruby/uri#107

lib/json_schemer.rb

@@ -104,12 +104,16 @@ class InvalidEcmaRegexp < StandardError; end
 
   WINDOWS_URI_PATH_REGEX = /\A\/[a-z]:/i
 
+  # :nocov:
+  URI_PARSER = URI.const_defined?(:RFC2396_PARSER) ? URI::RFC2396_PARSER : URI::DEFAULT_PARSER
+  # :nocov:
+
   FILE_URI_REF_RESOLVER = proc do |uri|
     raise InvalidFileURI, 'must use `file` scheme' unless uri.scheme == 'file'
     raise InvalidFileURI, 'cannot have a host (use `file:///`)' if uri.host && !uri.host.empty?
     path = uri.path
     path = path[1..-1] if path.match?(WINDOWS_URI_PATH_REGEX)
-    JSON.parse(File.read(URI::DEFAULT_PARSER.unescape(path)))
+    JSON.parse(File.read(URI_PARSER.unescape(path)))
   end
 
   class << self
@@ -247,7 +251,7 @@ def resolve(schema, options)
       when String
         JSON.parse(schema)
       when Pathname
-        base_uri = URI.parse(File.join('file:', URI::DEFAULT_PARSER.escape(schema.realpath.to_s)))
+        base_uri = URI.parse(File.join('file:', URI_PARSER.escape(schema.realpath.to_s)))
         options[:base_uri] = base_uri
         if options.key?(:ref_resolver)
           FILE_URI_REF_RESOLVER.call(base_uri)

test/output_format_test.rb

@@ -154,7 +154,7 @@ def test_it_escapes_absolute_keyword_location
     )
 
     uri = URI(absolute_keyword_location)
-    json_pointer = URI::DEFAULT_PARSER.unescape(uri.fragment)
+    json_pointer = JSONSchemer::URI_PARSER.unescape(uri.fragment)
     assert_equal(
       '/patternProperties/^(a[b]{2}c|#%z\\"<>`|[\\-_.!~0*\'();~1?:@&=+$,])/not',
       json_pointer