Limit object allocations

[ThomasSevestre]

This is an attempt to fix #111

[md5]

In the case that prefix has not been set and there are namespaces that are not SPREADSHEETML_URI, the code is still going to repeatedly set the name_* variables.

Perhaps this would make sense?

Suggested change

	if prefix.empty? && node.namespaces.any?
	namespace = node.namespaces.detect{|_key, uri| uri == SPREADSHEETML_URI }
	if prefix.empty? && (namespace = node.namespaces.detect{|_key, uri| uri == SPREADSHEETML_URI }).present?

[md5]

I guess that suggestion leaves some quirks around the if namespace && ... check below...

[ThomasSevestre]

In the case that prefix has not been set and there are namespaces that are not SPREADSHEETML_URI, the code is still going to repeatedly set the name_* variables.

Perhaps this would make sense?

I don't know if this is a common usecase.

If it is, we need to track prefix changes in order to handle new prefix and prefix reset :

if prefix != old_prefix
  name_row = "#{prefix}row"
  name_c = "#{prefix}c"
  name_v = "#{prefix}v"
  name_t = "#{prefix}t"
end

[pythonicrubyist]

Will test and merge tonight. Thanks