Show XML parsing errors when using IO as input.

Previously this wouldn't display anything due to the IO object being exhausted.
To fix this the input has to be wound back to the start, which means re-reading
it. Sadly I can't think of a way around this that doesn't require buffering
lines while parsing them (which massively increases memory usage).

lib/oga/xml/parser.y

@@ -230,16 +230,21 @@ end
   def on_error(type, value, stack)
     name        = token_to_str(type)
     index       = @line - 1
-    lines = @data.each_line.to_a
+    index_range = (index - 5)..(index + 5)
     code        = ''
 
-    # Show up to 5 lines before and after the offending line (if they exist).
-    (-5..5).each do |offset|
-      line   = lines[index + offset]
-      number = @line + offset
+    # For IO we sadly have to re-read the input :<
+    if @data.respond_to?(:rewind)
+      @data.rewind
+    end
 
-      if line and number > 0
-        if offset == 0
+    # Show up to 5 lines before and after the offending line (if they exist).
+    @data.each_line.with_index do |line, line_index|
+      next unless index_range.cover?(line_index)
+
+      number = line_index + 1
+
+      if line_index == index
         prefix = '=> '
       else
         prefix = '   '
@@ -253,7 +258,6 @@ end
 
       code << "#{prefix}#{number}: #{line}\n"
     end
-    end
 
     raise Racc::ParseError, <<-EOF.strip
 Unexpected #{name} with value #{value.inspect} on line #{@line}:

spec/oga/xml/parser/error_spec.rb

@@ -16,7 +16,7 @@ describe Oga::XML::Parser do
       expect { parse(@invalid_xml) }.to raise_error(Racc::ParseError)
     end
 
-    example 'display a more meaningful error message' do
+    example 'include the offending input when using String as input' do
       # Racc basically reports errors at the last moment instead of where they
       # *actually* occur.
       partial = <<-EOF.strip
@@ -29,5 +29,19 @@ describe Oga::XML::Parser do
 
       parse_error(@invalid_xml).should =~ /#{partial}/
     end
+
+    example 'include the offending input when using IO as input' do
+      # Racc basically reports errors at the last moment instead of where they
+      # *actually* occur.
+      partial = <<-EOF.strip
+   1. <person>
+   2. <name>Alice</name>
+   3. <age>25
+   4. <nationality>Dutch</nationality>
+=> 5. </person>
+      EOF
+
+      parse_error(StringIO.new(@invalid_xml)).should =~ /#{partial}/
+    end
   end
 end