Build self_hash up from INLINE_STYLES

Iterate over each value in INLINE_STYLES instead of iterating over
each value in instances_values and rejecting unwanted items.  This
version proceses fewer values and runs a little faster.

Issue randym#61 - Axlsx performance

lib/axlsx/workbook/worksheet/cell.rb

@@ -225,7 +225,8 @@ def initialize(row, value="", options={})
     # equality comparison to test value, type and inline style attributes
     # this is how we work out if the cell needs to be added or already exists in the shared strings table
     def shareable_hash
-      self_hash = self.instance_values.reject { |key, val| !INLINE_STYLES.include?(key) }
+      self_hash = {}
+      INLINE_STYLES.each { |style| self_hash[style] = self.instance_variable_get("@" + style) }
       self_hash['color'] = self_hash['color'].instance_values if self_hash['color']
       self_hash
     end