Add Source#skip_spaces? method

## Why?
In the case of `@source.match?(/\s+/um, true)`, if there are no spaces at the beginning, I want to stop reading immediately.
However, it continues to read the buffer until it finds a match, but it never finds a match.
As a result, it continues reading until the end of the file.

In the case of large XML files, drop_parsed_content occur frequently until the buffer is cleared, which may affect performance.

Author: naitoh

lib/rexml/parsers/baseparser.rb

@@ -280,7 +280,7 @@ def pull_event
               return [ :comment, process_comment ]
             elsif @source.match?("DOCTYPE", true)
               base_error_message = "Malformed DOCTYPE"
-              unless @source.match?(/\s+/um, true)
+              unless @source.skip_spaces?
                 if @source.match?(">")
                   message = "#{base_error_message}: name is missing"
                 else
@@ -391,7 +391,7 @@ def pull_event
               return [ :attlistdecl, element, pairs, contents ]
             elsif @source.match?("NOTATION", true)
               base_error_message = "Malformed notation declaration"
-              unless @source.match?(/\s+/um, true)
+              unless @source.skip_spaces?
                 if @source.match?(">")
                   message = "#{base_error_message}: name is missing"
                 else
@@ -735,7 +735,7 @@ def process_comment
 
       def process_instruction
         name = parse_name("Malformed XML: Invalid processing instruction node")
-        if @source.match?(/\s+/um, true)
+        if @source.skip_spaces?
           match_data = @source.match(/(.*?)\?>/um, true)
           unless match_data
             raise ParseException.new("Malformed XML: Unclosed processing instruction", @source)

lib/rexml/source.rb

@@ -65,6 +65,7 @@ class Source
     attr_reader :encoding
 
     module Private
+      SKIP_SPACES = /\s+/um
       SCANNER_RESET_SIZE = 100000
       PRE_DEFINED_TERM_PATTERNS = {}
       pre_defined_terms = ["'", '"', "<", "]]>"]
@@ -150,6 +151,10 @@ def match?(pattern, cons=false)
       end
     end
 
+    def skip_spaces?
+      @scanner.skip(Private::SKIP_SPACES) ? true : false
+    end
+
     def position
       @scanner.pos
     end

test/parse/test_document_type_declaration.rb

@@ -49,10 +49,10 @@ def test_no_name
         end
         assert_equal(<<-DETAIL.chomp, exception.to_s)
 Malformed DOCTYPE: name is missing
-Line: 3
-Position: 17
+Line: 1
+Position: 10
 Last 80 unconsumed characters:
-<!DOCTYPE>  <r/> 
+<!DOCTYPE>
         DETAIL
       end
     end