CDK-761: Fix inferred schema data samples.

This truncates values to 50 characters and removes all but a white list
of printable characters. Unicode characters will be replaced with '.' in
sample data for compatibility with the Hive MetaStore.
This is missing + and `. It would be simpler to use ASCII ranges:

```java
CharMatcher.inRange('\u0020', '\u007e').negate()
```