For my current project I needed to rework BigFile interface to support on-server data appension. But while doing so I've discovered in many places current BigFile code could crash and is not working properly. Thus this patch series contain:

1) fixes for discovered bugs;
2) support for on-server data append via exposing ._dataAppend(data_chunk) method;
3) basic tests for BigFile (so that we know fixes are proper and we won't regress on
   the same things in the future).

Newly introduced BigFile tests were verified to pass on the testrunner:

https://nexedi.erp5.net/test_result_module/20150303-3789A033 https://nexedi.erp5.net/test_result_module/20150303-3789A033/7

NOTE running the whole testsuite failed because of:

• erp5_test_result:testTaskDistribution failure https://nexedi.erp5.net/test_result_module/20150303-3789A033/23 .

testTaskDistribution is currently failing from time-to-time even on ERP5-MASTER tests, e.g. here https://nexedi.erp5.net/test_result_module/20150226-1F4459D0, so from my point of view that should not be related to BigFile at all.

Please pull.

Thanks beforehand, Kirill

Changes since first pull request (https://lab.nexedi.cn/nexedi/erp5/merge_requests/2)

• Renamed GET/PUT/CHECK -> get/put/check as per JP comment;
• Renamed _308 -> R308 as per JP comment;
• In tabular tests expanded HTTP header names to plain strings, so now shortcuts for their names are not used (as per input from Seb and Romain);
• Added note for _testBigFile_02_DataVarious that it is "called from under testBigFile_02_DataVarious driver" (as per question from Jérome).