Removing syntax constructs unsupported in rspec 3

- be_false/be_true matchers replaced
- When expecting block not to throw expection, arguments cannot be used. I.e., any expection violates the expectation.
- have() matcher unsupported. Replaced with .count() method.
Fixes EGI-FCTF/rOCCI-core#10, attrs in headers

Added support for Rack-compliant HTTP headers in Occi::Parser.
The following representations are now supported:
* X-OCCI-Attribute
* X_OCCI_ATTRIBUTE
* HTTP_X_OCCI_ATTRIBUTE
Fixes EGI-FCTF/rOCCI-core#9, link parsing

References to resource links are now correctly added to resources
within a collection.
Fixes EGI-FCTF/rOCCI-core#5, attr rendering

Attribute values containing strings with commas are no longer
mangled by `to_string`. This affects mostly links with inline
attributes.
Minor version bump to 4.2.15