fix default charset issue at `StaticHandlerTest#testContentHeadersSet` run test under non UTF-8 as default charset , will reproduce the problem ie: `mvn test -Dtest=StaticHandlerTest#testContentHeadersSet -Dfile.encoding=GBK` ==> ``` Starting test: StaticHandlerTest#testContentHeadersSet org.junit.ComparisonFailure: expected:<text/html;charset=[UTF-8]> but was:<text/html;charset=[GBK]>         at org.junit.Assert.assertEquals(Assert.java:115)         at org.junit.Assert.assertEquals(Assert.java:144)         at io.vertx.test.core.AsyncTestBase.assertEquals(AsyncTestBase.java:354)         at io.vertx.ext.web.handler.StaticHandlerTest.lambda$testContentHeadersSet$1(StaticHandlerTest.java:147) ```
fix default charset issue at `StaticHandlerTest#testContentHeadersSet`
run test under non UTF-8 as default charset , will reproduce the problem
ie: `mvn test -Dtest=StaticHandlerTest#testContentHeadersSet -Dfile.encoding=GBK`
==>
```
Starting test: StaticHandlerTest#testContentHeadersSet
org.junit.ComparisonFailure: expected:<text/html;charset=[UTF-8]> but was:<text/html;charset=[GBK]>
        at org.junit.Assert.assertEquals(Assert.java:115)
        at org.junit.Assert.assertEquals(Assert.java:144)
        at io.vertx.test.core.AsyncTestBase.assertEquals(AsyncTestBase.java:354)
        at io.vertx.ext.web.handler.StaticHandlerTest.lambda$testContentHeadersSet$1(StaticHandlerTest.java:147)
```

vert-x3/vertx-web,vert-x3/vertx-web,aesteve/vertx-web,InfoSec812/vertx-web,aesteve/vertx-web,aesteve/vertx-web,InfoSec812/vertx-web,InfoSec812/vertx-web,vert-x3/vertx-web,aesteve/vertx-web,InfoSec812/vertx-web,vert-x3/vertx-web,InfoSec812/vertx-web,aesteve/vertx-web,vert-x3/vertx-web,InfoSec812/vertx-web