PDI-9555 - Webservice via /kettle/executeTrans - Pass output to servlet
option: getServletPrintWriter does not respect UTF-8 encoding

What was done:
1) added new variable KETTLE_DEFAULT_SERVLET_ENCODING
2) changed the way PrintWriter for servlet output is instantionated.
3) changed the way Trans creates servletPrintWriter
PDI-11319 - fix - IfNull step 'Set empty string' forward incompatibility
PDI-11319 - unit test - IfNull step 'Set empty string' forward incompatibility
Merge pull request #355 from dbahdano/PDI-11319

PDI-11319 IfNull step "Set empty string" forward incompatibility
Merge pull request #316 from kolinus/PDI-9555

PDI-9555 - Webservice via /kettle/executeTrans - Pass output to servlet option: getServletPrintWriter does not respect UTF-8 encoding
PDI-10250 - "Group by" step - Minimum aggregation doesn't work

Fix provided.
2 new variables introduced: KETTLE_AGGREGATION_MIN_NULL_IS_VALUED and
KETTLE_AGGREGATION_ALL_NULLS_ARE_ZERO.
junits added.
PDI-6960 - Group by and Memory group by ignores NULL values ("sum" aggregation)

Do not fill with 0 by default new aggregations.
If aggregation still null - this will return null or 0 depends on 'KETTLE_AGGREGATION_ALL_NULLS_ARE_ZERO' variable.
Junits added.
PDI-11530 - "Memory Group by" step - Minimum aggregation doesn't work

fix added.
junit tests added.
PDI-9662,11597 junit aggregation tests
PDI-9662 - Denormalizer step: Minimum aggregation doesn't work

fix.
Avoid newGroup() intialize min with null.
Added support of new varibles to handle aggregation null-cases.
PDI-11597 - Denormalizer step: 100 + null = null

fix.
replace ValueDataUtil.plus(...) call with sum(...) to handle nulls
properly.