Allow setting ssl certs and keys as strings

At the moment Savon has ssl authentications parameters ssl_cert_file, ssl_cert_key_file, ssl_ca_cert_file. Thus, it is required to store these data in files. On the other hand, it may be sometimes convenient to store certificates, say, in a database column.

Since HTTPI provides API to set ssl certificates as strings, it seems to be possible to implement similar params in Savon, like following:

```ruby
# Class user has fields cert and cert_key that store corresponding strings 
user = User.find(params[:id)

Savon.client(
  ssl_cert: user.cert,
  ssl_cert_key: user.cert_key,
  ssl_ca_cert_file: "lib/ca_cert.pem"
)
```