Modify withFirst() to include (first, first) pair into result

[amaembo]

Currently withFirst() operation does not include (first, first) pair into result, so when one-element stream is passed, the resulting stream is empty. This limits the number of applications for this operation. A proposed change is to include (first, first) pair into result. To mimic the current behavior, withFirst().skip(1) could be used. Unfortunately this would introduce behavioral incompatibility with current version. Hopefully withFirst() is not widely used yet, so this would be an acceptable change.