Pipe output directly to str and avoid additional memory allocations

Currently, there are lots of examples of code like this,
```
str << ('<tag ' << foo << ' ' << bar << '/>')
```
which create the string for the tag in memory before piping to str.

We can avoid creating all of these intermediate strings by dropping
the paranthesis and piping directly to str.

This relies on the `str` passed around to handle lots of small
appends. This is a problem when using RubyZip, but that is solved
in the next commit.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/axlsx/drawing/pie_series.rb b/lib/axlsx/drawing/pie_series.rb
index 52cd2b66..f3edd1e9 100644
--- a/lib/axlsx/drawing/pie_series.rb
+++ b/lib/axlsx/drawing/pie_series.rb
@@ -49,9 +49,9 @@ module Axlsx
         str << '<c:explosion val="' + @explosion.to_s + '"/>' unless @explosion.nil?
         colors.each_with_index do |c, index|
           str << '<c:dPt>'
-          str << (+'<c:idx val="' << index.to_s << '"/>')
+          str << '<c:idx val="' << index.to_s << '"/>'
           str << '<c:spPr><a:solidFill>'
-          str << (+'<a:srgbClr val="' << c << '"/>')
+          str << '<a:srgbClr val="' << c << '"/>'
           str << '</a:solidFill></c:spPr></c:dPt>'
         end
         @labels.to_xml_string str unless @labels.nil?