Supports multi-line constant body with escaped nl

In parse_constant_body method,
TkNL is used as terminator of constant body,
but TkBACKSLASH can be escape character for TkNL in Ruby syntax.
This commit fixes it.

Author: aycabta

lib/rdoc/parser/ruby.rb

@@ -927,6 +927,8 @@ def parse_constant_body container, constant # :nodoc:
         end
       when TkCOLON2, TkCOLON3 then
         rhs_name << '::'
+      when TkBACKSLASH then
+        get_tk if TkNL === peek_tk
       when nil then
         break
       end

test/test_rdoc_parser_ruby.rb

@@ -2286,6 +2286,9 @@ class Foo
   SIXTH_CONSTANT = #{sixth_constant}
 
   SEVENTH_CONSTANT = proc { |i| begin i end }
+
+  EIGHTH_CONSTANT = "a" \\
+                    "b"
 end
 EOF
 
@@ -2331,6 +2334,11 @@ class Foo
     assert_equal 'SEVENTH_CONSTANT', constant.name
     assert_equal "proc { |i| begin i end }", constant.value
     assert_equal @top_level, constant.file
+
+    constant = constants[7]
+    assert_equal 'EIGHTH_CONSTANT', constant.name
+    assert_equal "\"a\" \\\n\"b\"", constant.value
+    assert_equal @top_level, constant.file
   end
 
   def test_parse_statements_identifier_attr