change blockchain -> block chain (spelling)

- Wiki says "block chain" is correct ;)
- remove some unneeded spaces I found in the source, while fixing the spelling

src/test/DoS_tests.cpp

@@ -129,7 +129,6 @@ BOOST_AUTO_TEST_CASE(DoS_checknbits)
 
     // ... but OK if enough time passed for difficulty to adjust downward:
     BOOST_CHECK(CheckNBits(firstcheck.second, lastcheck.first+60*60*24*365*4, lastcheck.second, lastcheck.first));
-    
 }
 
 CTransaction RandomOrphan()