Corrected as_leading_term in sinh, tanh and coth

Replaced the output with x, x and 1/x respectively and added tests.
as_leading_term method of acosh and acoth corrected

Output changed to I*pi/2, tests added and similar tests
put together to save lines.
Merge pull request #8456 from shivamvats/sinh

Corrected as_leading_term in hyperbolic functions (fixes #8455 )