Use StringIO to remove duplication

Previously we had to separate branches which did the same thing, one was
for IOs and other was for strings. However, if we wrap the string into a
StringIO, we can use the same branch for both.

lib/mimemagic.rb

 require 'mimemagic/tables'
 require 'mimemagic/version'
 
+require 'stringio'
+
 # Mime type detection
 class MimeMagic
   attr_reader :type, :mediatype, :subtype
@@ -113,8 +115,7 @@ class MimeMagic
       MAGIC.send(method) { |type, matches| magic_match_io(io, matches) }
     else
       str = io.respond_to?(:read) ? io.read : io.to_s
-      str = str.force_encoding(Encoding::BINARY) if str.respond_to?(:force_encoding)
-      MAGIC.send(method) { |type, matches| magic_match_str(str, matches) }
+      magic_match(StringIO.new(str), method)
     end
   end
 
@@ -133,18 +134,5 @@ class MimeMagic
     end
   end
 
-  def self.magic_match_str(str, matches)
-    matches.any? do |offset, value, children|
-      match =
-        if Range === offset
-          x = str[offset.begin, offset.end - offset.begin + value.bytesize]
-          x && x.include?(value)
-        else
-          str[offset, value.bytesize] == value
-        end
-      match && (!children || magic_match_str(str, children))
-    end
-  end
-
-  private_class_method :magic_match, :magic_match_io, :magic_match_str
+  private_class_method :magic_match, :magic_match_io
 end