Fix FastCGI server hanging and not serving any requests

Commit baae1b64a5b9685a32425434e33fe3f00d015b4d, which refactored some code, did not correctly refactor the data_missing field.  The field [had an initializer of true](https://github.com/mono/xsp/commit/baae1b64a5b9685a32425434e33fe3f00d015b4d#L0L95), but the new auto-property was never initialized, and so it had the default value of false.  This results in the FastCGI server never responding to requests, in my case leading to nginx returning "504 Gateway time-out".