Predicates with not qualifier

[fabricereix]

The not qualifier in the predicate negates most of the time the predicate result.

But there are a few cases, for which it does not invert a predicate failure.
Hurl detects some inconsistencies between the predicate and the value to which it is applied and prefer to make the assert fail.

Example 1 - the value (returned by the query) is 1 (integer)

All the following predicates fail:

equals 2
equals null
contains "toto"

Negating the predicate does not necessarily make the predicate succeeds.

not equals 2       
not equals null  
not contains "toto" => failure

The last one fails because the contains predicate only apply to String value, and thus it does not make sense to apply it to a integer value.
This is equivalent to a type error.

Example 2 - the value is not found

All the following predicates fail:

exist
equals 1
contains "toto"

all these predicates will success if there are negated

not exist
not equals 1
not contains "toto"

There can't be any "type checking" when no value is returned.

Example 3 - the value is present but does not have a value

for example, the Secure cookie attribute, it is present or not
but it does not have a value

All the following predicates fail:

equals true
equals false
equals 1
contains "toto"

All these predicates will fail even if they are negated.

not equals true
not equals false
not equals 1
not contains "toto"

They must be applied to a value.
exist is the only predicate that can be applied.

[fabricereix]

for predicate success, not will always negate as predicate failure

[fabricereix]

implemented in #43