add default value to summary

The proposal are reduce the number of methods to get a summary, considering that not make sense have two filters when it was already defined in summarizing Monetary method, so:
from: 
      
      MonetarySummaryStatistics summary = getCurrencies().stream()
				.filter(MonetaryFunctions.isCurrency(DOLLAR))
				.collect(MonetaryFunctions.summarizingMonetary(DOLLAR));

to: 

     MonetarySummaryStatistics summary = getCurrencies().stream()
				.collect(MonetaryFunctions.summarizingMonetary(DOLLAR));


Another changes was the creation of a decoration to group summary, this way when a currency that was not summarizing, now will return an empty summary instead of a null.