Mock plugin single byte read returns -1 for 0xff.

[gugrim]

The single byte read method in MockI2C returns an unmasked byte as its int result, i e
return b;
instead of
return b & 0xff;

Byte values from 0x80 to 0xff are returned as negative ints making it impossible to test for EOF.

Diff:
`diff --git a/plugins/pi4j-plugin-mock/src/main/java/com/pi4j/plugin/mock/provider/i2c/MockI2C.java b/plugins/pi4j-plugin-mock/src/main/java/com/pi4j/plugin/mock/provider/i2c/MockI2C.java
index f161576..bfad59a 100644
--- a/plugins/pi4j-plugin-mock/src/main/java/com/pi4j/plugin/mock/provider/i2c/MockI2C.java
+++ b/plugins/pi4j-plugin-mock/src/main/java/com/pi4j/plugin/mock/provider/i2c/MockI2C.java
@@ -155,7 +155,7 @@ public class MockI2C extends I2CBase implements I2C {
logger.info("] :: READ(0x");
logger.info(StringUtil.toHexString(b));
logger.info(")");

•    return b;
  

•    return b & 0xff;
  

  }

  /** {@inheritdoc} */`